MDEV-8785 Wrong results for EXPLAIN EXTENDED...WHERE NULLIF(latin1_col, _utf8'a' COLLATE utf8_bin) IS NOT NULL

Author: Alexander Barkov

mysql-test/r/explain_json.result

@@ -1090,3 +1090,23 @@ EXPLAIN
  }
 }
 DROP TABLE t1;
+#
+# MDEV-8785 Wrong results for EXPLAIN EXTENDED...WHERE NULLIF(latin1_col, _utf8'a' COLLATE utf8_bin) IS NOT NULL
+#
+CREATE TABLE t1 (a VARCHAR(10) CHARACTER SET latin1);
+INSERT INTO t1 VALUES ('a'),('A');
+EXPLAIN FORMAT=JSON SELECT * FROM t1 WHERE NULLIF(a,_utf8'a' COLLATE utf8_bin);
+EXPLAIN
+{
+ "query_block": {
+ "select_id": 1,
+ "table": {
+ "table_name": "t1",
+ "access_type": "ALL",
+ "rows": 2,
+ "filtered": 100,
+ "attached_condition": "(case when convert(t1.a using utf8) = <cache>((_utf8'a' collate utf8_bin)) then NULL else t1.a end)"
+ }
+ }
+}
+DROP TABLE t1;

mysql-test/r/null.result

@@ -1390,5 +1390,24 @@ Warning	1292	Incorrect datetime value: '1'
 Warning	1292	Incorrect datetime value: '1'
 DROP TABLE t1;
 #
+# MDEV-8785 Wrong results for EXPLAIN EXTENDED...WHERE NULLIF(latin1_col, _utf8'a' COLLATE utf8_bin) IS NOT NULL
+#
+CREATE TABLE t1 (a VARCHAR(10) CHARACTER SET latin1);
+INSERT INTO t1 VALUES ('a'),('A');
+SELECT a, NULLIF(a,_utf8'a' COLLATE utf8_bin) IS NULL FROM t1;
+a	NULLIF(a,_utf8'a' COLLATE utf8_bin) IS NULL
+a	1
+A	0
+SELECT CHARSET(NULLIF(a,_utf8'a' COLLATE utf8_bin)) FROM t1;
+CHARSET(NULLIF(a,_utf8'a' COLLATE utf8_bin))
+latin1
+latin1
+EXPLAIN EXTENDED SELECT NULLIF(a,_utf8'a' COLLATE utf8_bin) IS NULL AS expr FROM t1;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
+1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	2	100.00
+Warnings:
+Note	1003	select isnull((case when convert(`test`.`t1`.`a` using utf8) = (_utf8'a' collate utf8_bin) then NULL else `test`.`t1`.`a` end)) AS `expr` from `test`.`t1`
+DROP TABLE t1;
+#
 # End of 10.1 tests
 #

mysql-test/t/explain_json.test

@@ -286,3 +286,11 @@ CREATE TABLE t1 (a VARCHAR(10) CHARACTER SET latin1);
 INSERT INTO t1 VALUES ('a'),('b');
 EXPLAIN FORMAT=JSON SELECT * FROM t1 WHERE a=_latin1 0xDF;
 DROP TABLE t1;
+
+--echo #
+--echo # MDEV-8785 Wrong results for EXPLAIN EXTENDED...WHERE NULLIF(latin1_col, _utf8'a' COLLATE utf8_bin) IS NOT NULL
+--echo #
+CREATE TABLE t1 (a VARCHAR(10) CHARACTER SET latin1);
+INSERT INTO t1 VALUES ('a'),('A');
+EXPLAIN FORMAT=JSON SELECT * FROM t1 WHERE NULLIF(a,_utf8'a' COLLATE utf8_bin);
+DROP TABLE t1;

mysql-test/t/null.test

@@ -868,6 +868,16 @@ CREATE TABLE t1 AS SELECT
  END AS b;
 DROP TABLE t1;
 
+--echo #
+--echo # MDEV-8785 Wrong results for EXPLAIN EXTENDED...WHERE NULLIF(latin1_col, _utf8'a' COLLATE utf8_bin) IS NOT NULL
+--echo #
+CREATE TABLE t1 (a VARCHAR(10) CHARACTER SET latin1);
+INSERT INTO t1 VALUES ('a'),('A');
+SELECT a, NULLIF(a,_utf8'a' COLLATE utf8_bin) IS NULL FROM t1;
+SELECT CHARSET(NULLIF(a,_utf8'a' COLLATE utf8_bin)) FROM t1;
+EXPLAIN EXTENDED SELECT NULLIF(a,_utf8'a' COLLATE utf8_bin) IS NULL AS expr FROM t1;
+DROP TABLE t1;
+
 
 --echo #
 --echo # End of 10.1 tests

sql/item_cmpfunc.cc

@@ -2677,6 +2677,70 @@ Item_func_nullif::fix_length_and_dec()
 }
 
 
+void Item_func_nullif::print(String *str, enum_query_type query_type)
+{
+ /*
+ NULLIF(a,b) is implemented according to the SQL standard as a short for
+ CASE WHEN a=b THEN NULL ELSE a END
+
+ The constructor of Item_func_nullif sets args[0] and m_args0_copy to the
+ same item "a", and sets args[1] to "b".
+
+ If "this" is a part of a WHERE or ON condition, then:
+ - the left "a" is a subject to equal field propagation with ANY_SUBST.
+ - the right "a" is a subject to equal field propagation with IDENTITY_SUBST.
+ Therefore, after equal field propagation args[0] and m_args0_copy can point
+ to different items.
+ */
+ if (!(query_type & QT_ITEM_FUNC_NULLIF_TO_CASE) || args[0] == m_args0_copy)
+ {
+ /*
+ If no QT_ITEM_FUNC_NULLIF_TO_CASE is requested,
+ that means we want the original NULLIF() representation,
+ e.g. when we are in:
+ SHOW CREATE {VIEW|FUNCTION|PROCEDURE}
+
+ The original representation is possible only if
+ args[0] and m_args0_copy still point to the same Item.
+
+ The caller must pass call print() with QT_ITEM_FUNC_NULLIF_TO_CASE
+ if an expression has undergone some optimization
+ (e.g. equal field propagation done in optimize_cond()) already and
+ NULLIF() potentially has two different representations of "a":
+ - one "a" for comparison
+ - another "a" for the returned value!
+
+ Note, the EXPLAIN EXTENDED and EXPLAIN FORMAT=JSON routines
+ do pass QT_ITEM_FUNC_NULLIF_TO_CASE to print().
+ */
+ DBUG_ASSERT(args[0] == m_args0_copy);
+ str->append(func_name());
+ str->append('(');
+ m_args0_copy->print(str, query_type);
+ str->append(',');
+ args[1]->print(str, query_type);
+ str->append(')');
+ }
+ else
+ {
+ /*
+ args[0] and m_args0_copy are different items.
+ This is possible after WHERE optimization (equal fields propagation etc),
+ e.g. in EXPLAIN EXTENDED or EXPLAIN FORMAT=JSON.
+ As it's not possible to print as a function with 2 arguments any more,
+ do it in the CASE style.
+ */
+ str->append(STRING_WITH_LEN("(case when "));
+ args[0]->print(str, query_type);
+ str->append(STRING_WITH_LEN(" = "));
+ args[1]->print(str, query_type);
+ str->append(STRING_WITH_LEN(" then NULL else "));
+ m_args0_copy->print(str, query_type);
+ str->append(STRING_WITH_LEN(" end)"));
+ }
+}
+
+
 /**
  @note
  Note that we have to evaluate the first argument twice as the compare

sql/item_cmpfunc.h

@@ -928,12 +928,7 @@ class Item_func_nullif :public Item_func_hybrid_field_type
  void fix_length_and_dec();
  uint decimal_precision() const { return m_args0_copy->decimal_precision(); }
  const char *func_name() const { return "nullif"; }
-
- virtual inline void print(String *str, enum_query_type query_type)
- {
- Item_func::print(str, query_type);
- }
-
+ void print(String *str, enum_query_type query_type);
  table_map not_null_tables() const { return 0; }
  bool is_null();
  Item* propagate_equal_fields(THD *thd, const Context &ctx, COND_EQUAL *cond)

sql/mysqld.h

@@ -654,9 +654,18 @@ enum enum_query_type
  /// If Item_subselect should print as just "(subquery#1)"
  /// rather than display the subquery body
  QT_ITEM_SUBSELECT_ID_ONLY= (1 << 5),
+ /// If NULLIF(a,b) should print itself as
+ /// CASE WHEN a_for_comparison=b THEN NULL ELSE a_for_return_value END
+ /// when "a" was replaced to two different items
+ /// (e.g. by equal fields propagation in optimize_cond()).
+ /// The default behaviour is to print as NULLIF(a_for_return, b)
+ /// which should be Ok for SHOW CREATE {VIEW|PROCEDURE|FUNCTION}
+ /// as they are not affected by WHERE optimization.
+ QT_ITEM_FUNC_NULLIF_TO_CASE= (1 <<6),
 
  /// This value means focus on readability, not on ability to parse back, etc.
  QT_EXPLAIN= QT_TO_SYSTEM_CHARSET |
+ QT_ITEM_FUNC_NULLIF_TO_CASE |
  QT_ITEM_IDENT_SKIP_CURRENT_DATABASE |
  QT_ITEM_CACHE_WRAPPER_SKIP_DETAILS |
  QT_ITEM_SUBSELECT_ID_ONLY,
@@ -665,7 +674,7 @@ enum enum_query_type
  /// Be more detailed than QT_EXPLAIN.
  /// Perhaps we should eventually include QT_ITEM_IDENT_SKIP_CURRENT_DATABASE
  /// here, as it would give better readable results
- QT_EXPLAIN_EXTENDED= QT_TO_SYSTEM_CHARSET
+ QT_EXPLAIN_EXTENDED= QT_TO_SYSTEM_CHARSET | QT_ITEM_FUNC_NULLIF_TO_CASE
 };